1. Technical Field
The present invention relates to a semiconductor, in particular, a transistor formed on silicon on insulator (SOI) substrate.
2. Related Art
Forming a transistor on a SOI substrate has an advantage of low power consumption and low voltage drive because of small parasitic capacitance, comparing with a case when a transistor is formed on a bulk substrate. Further, such forming has another advantage in that high speed driving can be available and the development of it has been attracted attention.
Improving characteristics of such transistors are studied in diverse ways. For example, JPA8-228145 discloses a technology of low power consumption at the time of high-speed operation and waiting in that a body portion of a at MOSFET in a logic circuit is made be a floating state, lowering the threshold voltage of a MOSFET and a bias voltage is applied to a body portion of a MOSFET in a power source switch, raising the threshold voltage of a MOSFET.
The inventor sharply studied the improvement of characteristics of a semiconductor device formed on a SOI substrate, in particular, reducing leak current in a integrated circuit formed on a SOI substrate.
An inverter circuit (an inverter) is the most basic circuit block among integrated circuits, having a structure in that a n-channel type MISFET is connected to a p-channel type MISFET in series. In these inverters, a common gate of MOSFET becomes an input and a connecting portion between a n-channel type MOSFET and a p-channel type MOSFET becomes an output.
Further, in a SOI substrate in which multiple layers of an insulating film and a semiconductor film are deposited on a semiconductor substrate, it is important to give a fixed (control) potential not only to a body portion (a semiconductor film), but to a back gate portion (a semiconductor substrate) located next to the above portion through an insulating film, differing from a bulk substrate.
Here, in order to fix a body portion as a predetermined potential, it is necessary to install a contact portion for supplying potential on a region in which elements are formed (a semiconductor film), increasing a circuit area.
Then, the inventor studied an inverter structure in which a body portion becomes a floating state and the potential of a back gate portion is controlled.
However, when the backdate portion (a semiconductor substrate) is fixed as a ground potential (Vss), the potential difference between a source potential (Vss) and the backdate potential becomes zero in a n-channel type MISFET. But the potential difference between a source potential (Vdd) and the backdate potential becomes −Vdd in a p-channel type MISFET. Hence, the threshold value of the p-channel type MISFET moves to a depletion side and leakage current increases (see FIG. 3.) On the other hand, when the backdate portion (a semiconductor substrate) is fixed as a source potential (Vdd), the threshold value of a n-channel type MISFET moves to a depletion side and leakage current increases (see FIG. 3.)